I suspect if you've never heard of it, you might not be a long time local up there. While I have not personally seen it, I have been shown pictures of it with the explaination that it's a HUGE rock almost in the middle of nowhere and not many trees around. he picture showed a Jeep on top of it and the rock extended out both ends of the picture so I sense that it was really huge. I'm not sure how it was accessed or where specifically it was located. (Unless of course I was being bullschitted which I don't think I was because of who showed me the picture almost 15+ years ago. I trust the guy).
It sounded like the acid rain dead area up north east of Wawa. I spent 3 days up there a few years back roaming the backroads for 30 plus Kilometers and never found anything that resembled the picture I saw.
There's a decent amount of little play areas and rock piles in and around the old mining operation east of town but probably not enough to warrant a full blown trip to the area. Unless of course a local (yourself) cared to show a group around. That area is part of the Canadian Shield so there's bound to be lots of rock outcroppings to have fun on.
jim-kb8ymf